not building on Debian 12

[niallp]

getting a few warning on compile:

fire8.c:126:18: warning: operation on ‘rf_c’ may be undefined [-Wsequence-point]
126 | rf_c = ++rf_c % MAXRAND;
| ^
fire8.c:135:26: warning: operation on ‘rf_c’ may be undefined [-Wsequence-point]
135 | rf_c = ++rf_c % MAXRAND;
| ^
fire8.c:195:18: warning: operation on ‘rf_c’ may be undefined [-Wsequence-point]
195 | rf_c = ++rf_c & MAXRAND;
| ^
fire8.c:205:26: warning: operation on ‘rf_c’ may be undefined [-Wsequence-point]
205 | rf_c = ++rf_c % MAXRAND;
| ^

and then an error linking:

/usr/bin/ld: /tmp/ccq96Ob7.o:(.bss+0x20): multiple definition of fb_finfo'; fb.o:(.bss+0xb0): first defined here /usr/bin/ld: /tmp/ccq96Ob7.o:(.bss+0x90): multiple definition of fb_vinfo'; fb.o:(.bss+0x10): first defined here
/usr/bin/ld: /tmp/ccq96Ob7.o:(.bss+0x130): multiple definition of `fb_fd'; fb.o:(.bss+0x0): first defined here

understood this is pretty old code but any hints appreciated ...